Marine Trematode Research Articles

Freshwater trematodes differ from marine trematodes in patterns connected with division of labor.

Prior research suggests that trematode rediae, a developmental stage of trematode parasites that reproduce clonally within a snail host, show evidence of division of labor (DOL). Single-species infections often have two morphologically distinct groups: small rediae, the 'soldiers', are active, aggressive, and do not appear to reproduce; large rediae, the 'reproductives', are larger, sluggish, and full of offspring. Most data supporting DOL come from trematodes infecting marine snails, while data from freshwater trematodes are more limited and generally do not supported DOL. The shorter lifespan typical of freshwater snails may partially explain this difference: defending a short-lived host at the expense of reproduction likely provides few advantages. Here, we present data from sixty-one colonies spanning twenty species of freshwater trematode exploring morphological and behavioral patterns commonly reported from marine trematodes believed to have DOL. Trematode rediae were obtained from sixty-one infected snails collected in central Vermont, USA. A portion of the COI gene was sequenced to make tentative species identifications ('COI species'). Samples of rediae were photographed, observed, and measured to look for DOL-associated patterns including a bimodal size distribution, absence of embryos in small rediae, and pronounced appendages and enlarged pharynges (mouthparts) in small rediae. Additional rediae were used to compare activity levels and likelihood to attack heterospecific trematodes in large vs. small rediae. Many of the tests for DOL-associated patterns showed mixed results, even among colonies of the same COI species. However, we note a few consistent patterns. First, small rediae of most colonies appeared capable of reproduction, and we saw no indication (admittedly based on a small sample size and possibly insufficient attack trial methodology) that small rediae were more active or aggressive. This differs from patterns reported from most marine trematodes. Second, the small rediae of most colonies had larger pharynges relative to their body size than large rediae, consistent with marine trematodes. We also observed that colonies of three sampled COI species appear to produce a group of large rediae that have distinctly large pharynges. We conclude that these freshwater species likely do not have a group of specialized non-reproductive soldiers because small rediae of at least some colonies in almost every species do appear to produce embryos. We cannot rule out the possibility that small rediae act as a temporary soldier caste. We are intrigued by the presence of rediae with enlarged pharynges in some species and propose that they may serve an adaptive role, possibly similar to the defensive role of small 'soldier' rediae of marine trematodes. Large-pharynx rediae have been documented in other species previously, and we encourage future efforts to study these large-pharynx rediae.

Molecular species delimitation of marine trematodes over wide geographical ranges: Schikhobalotrema spp. (Digenea: Haplosplanchnidae) in needlefishes (Belonidae) from the Pacific Ocean and Gulf of Mexico.

Geographical distribution plays a major role in our understanding of marine biodiversity. Some marine fish trematodes have been shown to have highly restricted geographical distributions, while some are known to occur over very wide ranges; however, very few of these wide distributions have been demonstrated genetically. Here, we analyse species of the genus Schikhobalotrema (Haplosplanchnidae) parasitizing beloniforms from the tropical west Pacific, the eastern Pacific and the Gulf of Mexico (GoM). We test the boundaries of these trematodes by integrating molecular and morphological data, host association, habitat of the hosts and geographical distribution, following a recently proposed and standardized delineation method for the recognition of marine trematode species. Based on the new collections, Schikhobalotrema huffmani is here synonymized with the type-species of the genus, Schikhobalotrema acutum; Sch. acutum is now considered to be widely distributed, from the GoM to the western Pacific. Additionally, we describe a new species, Schikhobalotrema minutum n. sp., from Strongylura notata and Strongylura marina (Belonidae) from La Carbonera coastal lagoon, northern Yucatán, GoM. We briefly discuss the role of host association and historical biogeography of the hosts as drivers of species diversification of Schikhobalotrema infecting beloniforms.

Thermal optima of cercarial emergence in trematodes from a marine high-temperature ecosystem, the Persian Gulf

Global warming may alter the dynamics of infectious diseases by affecting important steps in the transmission of pathogens and parasites. In trematode parasites, the emergence of cercarial stages from their hosts is temperature-dependent, being highest around a thermal optimum. If environmental temperatures exceed this optimum as a consequence of global warming, this may affect cercarial transmission. However, our knowledge of cercarial emergence patterns of species from high temperature environments is currently very limited. Here, we investigated the effect of temperature on the emergence of two common trematode species from an abundant mud snail Pirenella cingulata in the Persian Gulf, the warmest sea on Earth. Infected snails were incubated in the laboratory at 6 temperatures from 10 to 40 °C for 3 days. We found an optimal temperature for cercarial emergence of 32.0 °C and 33.5 °C for Acanthotrema tridactyla and Cyathocotylidae gen. sp., respectively, which are the warmest recorded thermal optima for any aquatic trematode species. Emergence of both species dropped at 40 °C, suggesting upper thermal limits to emergence. Overall, Persian Gulf trematodes may be among the most heat-tolerant marine trematode species, indicating a potential for dispersing to regions that will continue to warm in the future.

Marine trematode parasites as indicators of environmental changes

Coastal ecosystems are threatened by growing pressures related to global change. The evaluation of an ecosystem’s biodiversity and status is by consequence of increasing importance to assess potentially undergone changes and predict their future trajectory for management and conservancy purposes. Biotic indicators sensitive to ecosystem changes are thus continuously sought for. Trematode parasite communities were examined as potential indicators of environmental changes, considering that the achievement of their complex life cycle is modulated by several factors. Indeed, the rule for trematode is that each species complete their cycle by a succession of three different host species and two free-living stages, thus depending on several biotic and abiotic factors. To this end, we examined the trematode community infecting the common cockle in Banc d’Arguin, Arcachon Bay, France. Monthly data extending over 16 years and sampled at a single station were compared to field book notes describing the changes of the closely surrounding landscape. In 2021, we also sampled cockles at 15 stations presenting different substrate features along the bank. Over time, seven out of nine changes of the trematode community structure presented concordances with changes in landscape. We hypothesize this was related to the environment heterogeneous substrate (i.e., temporal succession of oyster parks, bare sands and seagrass) with cascading effects on host populations. However, some changes could not solely be explained by changes in landscape. Our spatial study showed that the trematode communities exhibited intricate infection patterns with a complex interaction between substrate heterogeneity and larval dispersal ability of parasites. Thus, trematode communities might be potential indicators of subtle changes in the environment. However, it remains unclear which scale of environmental changes trematodes are actually sensitive to.

Host and geographic differences in prevalence and diversity of gastrointestinal helminths of foxes (Vulpes vulpes), coyotes (Canis latrans) and wolves (Canis lupus) in Québec, Canada.

Wild canids are hosts to a wide range of parasites and can play a role in transmission of zoonoses. As many parasites are transmitted through food webs, and wild canids are at high trophic levels, parasite prevalence and diversity in wild canids can serve as excellent indicators of ecosystem health. Our main objectives were to update knowledge on the composition of gastrointestinal helminths in wild canids from Québec, Canada, and to describe differences in parasite prevalence and diversity among canid species and regions. Hunters and trappers provided whole carcasses of red foxes (Vulpes vulpes) (N = 176), and intestinal tracts of coyotes (Canis latrans) (N = 77) and gray wolves (Canis lupus) (N = 23) harvested for non-research purposes over the winter of 2016–2017. A modified Stoll's centrifugation sucrose flotation on feces of 250 wild canids was used, and eggs of one family and eight genera of parasitic helminths were recovered: diphyllobothriids, Taenia/Echinococcus spp., Capillaria spp., Toxascaris sp., Toxocara sp., Trichuris sp., Uncinaria sp., and Metorchis sp. Adult Taenia spp. cestodes were recovered from 61 of 276 (22%) canids. Six different species (T. hydatigena, T. twitchelli, T. crassiceps, T. polyacantha, T. krabbei, and T. pisiformis-“like”) were differentiated based on DNA sequenced from 65 individual adult cestodes using primers for the nicotinamide adenosine dinucleotide dehydrogenase subunit 1 (ND1) and cytochrome c oxidase subunit 1 (CO1) mitochondrial DNA loci. Alaria sp. trematodes infected 89 of 276 canids (32%). A subset were identified as A. americana at the CO1 locus. The marine trematode Cryptocotyle lingua was reported for the first time in foxes in the province of Québec. These results help us understand more fully the predator-prey relationships within this group of canids. This baseline data in regional parasite prevalence and intensity is critical in order to detect future changes following ecological disturbances due to climate and landscape alterations.

Freshening rather than warming drives trematode transmission from periwinkles to mussels

In the Western Baltic Sea, climate change is happening at much faster rate than in most other seas and organisms are additionally exposed to a steep and variable salinity gradient. Climate change has previously been shown to affect parasite transmission in other marine ecosystems, yet little is known about potential effects of warming and desalination on parasite–host interactions. In laboratory experiments, we determined the combined effects of projected seawater warming and freshening on the emergence, activity, survival, and infectivity of cercariae (free-swimming infectious stage) of the trematode Himasthla elongata (Mehlis 1831), shed from its first intermediate host, the periwinkle Littorina littorea (Linnaeus 1758), in the Baltic Sea. We also assessed the susceptibility of the second intermediate host, the mussel Mytilus edulis Linnaeus, 1758, to cercarial infections. Generally, salinity was the main driver, particularly of cercarial activity, infectivity, and mussel susceptibility to infection. At the lowest salinity (13), cercariae were 50% less active compared to the highest salinity (19). Infection success and host susceptibility followed a similar pattern, with 47% and 43% less metacercariae (encysted stage) present at salinity 13 than at salinity 19, respectively. In contrast, effects of simulated warming were found only for cercarial survival, with cercarial longevity being higher at 19 than at 23 °C. No significant interactions between temperature and salinity were found. In contrast to the literature, the results suggest that a climate change-driven freshening (partly also warming) may lead to a general decline of marine trematodes, with possible beneficial effects for the involved hosts.

Complex and interactive effects of ocean acidification and warming on the life span of a marine trematode parasite

Human activities have caused an increase in atmospheric CO2 over the last 250 years, leading to unprecedented rates of change in seawater pH and temperature. These global scale processes are now commonly referred to as ocean acidification and warming, and have the potential to substantially alter the physiological performance of many marine organisms. It is vital that the effects of ocean acidification and warming on marine organisms are explored so that we can predict how marine communities may change in future. In particular, the effect of ocean acidification and warming on host-parasite dynamics is poorly understood, despite the ecological importance of these relationships. Here, we explore the response of one himasthlid trematode, Himasthla sp., an abundant and broadly distributed species of marine parasite, to combinations of elevated temperature and pCO2 that represent physiological extremes, pre-industrial conditions, and end of century predictions. Specifically, we quantified the life span of the free-living cercarial stage under elevated temperature and pCO2, focussing our research on functional life span (the time cercariae spend actively swimming) and absolute life span (the period before death). We found that the effects of temperature and pCO2 were complex and interactive. Overall, increased temperature negatively affected functional and absolute life span, e.g. across all pCO2 treatments the average time to 50% cessation of active swimming was approximately 8 h at 5 °C, 6 h at 15 °C, 4 h at 25 °C, and 2 h at 40 °C. The effect of pCO2, which significantly affected absolute life span, was highly variable across temperature treatments. These results strongly suggest that ocean acidification and warming may alter the transmission success of trematode cercariae, and potentially reduce the input of cercariae to marine zooplankton. Either outcome could substantially alter the community structure of coastal marine systems.

Temperature and pCO2 jointly affect the emergence and survival of cercariae from a snail host: implications for future parasitic infections in the Humboldt Current system

Ocean warming and acidification are general consequences of rising atmospheric CO2 concentrations. In addition to future predictions, highly productive systems such as the Humboldt Current System are characterized by important variations in both temperature and pCO2 level, but how these physical–chemical ocean changes might influence the transmission and survival of parasites has not been assessed. This study experimentally evaluated the effects of temperature (14, 18 and 25 °C) and the combined effects of temperature (∼15 and 20 °C) and pCO2 level (∼500 and 1400 microatmospheres (µatm) on the emergence and survival of two species of marine trematodes—Echinostomatidae gen. sp. and Philophthalmidae gen. sp.—both of which infect the intertidal snail Echinolittorina peruviana. Snails were collected from intertidal rocky pools in a year-round upwelling area of the northern Humboldt Current System (23°S). Two experiments assessed parasite emergence and survival by simulating emersion-immersion tidal cycles. To assess parasite survival, 2 h old cercariae (on average) were taken from a pool of infected snails incubated at 20–25 °C, and their mortality was recorded every 6 h until all the cercariae were dead. For both species, a trade-off between high emergence and low survival of cercariae was observed in the high temperature treatment. Species-specific responses to the combination of temperature and pCO2 levels were also observed: the emergence of Echinostomatidae cercariae was highest at 20 °C regardless of the pCO2 levels. By contrast, the emergence of Philophthalmidae cercariae was highest at elevated pCO2 (15 and 20 °C), suggesting that CO2 may react synergistically with temperature, increasing transmission success of this parasite in coastal ecosystems of the Humboldt Current System where water temperature and pH are expected to decrease. In conclusion, our results suggest that integrating temperature-pCO2 interactions in parasite studies is essential for understanding the consequence of climate change in future marine ecosystem health.

Consumer and host body size effects on the removal of trematode cercariae by ambient communities.

Parasite transmission can be altered via the removal of parasites by the ambient communities in which parasite-host interactions take place. However, the mechanisms driving parasite removal remain poorly understood. Using marine trematode cercariae as a model system, we investigated the effects of consumer and host body size on parasite removal rates. Laboratory experiments revealed that consumer or host body size significantly affected cercarial removal rates in crabs, oysters and cockles but not in shrimps. In general, cercarial removal rates increased with consumer (crabs and oysters) and host (cockles) body size. For the filter feeding oysters and cockles, the effects probably relate to their feeding activity which is known to correlate with bivalve size. Low infection levels found in cockle hosts suggest that parasite removal by hosts also leads to significant mortality of infective stages. The size effects of crab and shrimp predators on cercarial removal rates were more complex and did not show an expected size match-mismatch between predators and their cercarial prey, suggesting that parasite removal rates in predators are species-specific. We conclude that to have a comprehensive understanding of parasite removal by ambient communities, more research into the various mechanisms of cercarial removal is required.

Effect of dose and frequency of exposure to infectious stages on trematode infection intensity and success in mussels.

Marine parasites such as trematodes often compromise the fitness of their hosts. Such effects are generally considered to be density-dependent, i.e. the greater the infection intensity in the host, the greater the detrimental impact on host fitness. However, the mechanisms determining infection in marine hosts are still poorly understood. Here, we investigated the effect of cercarial dose and exposure frequency (single vs. trickle infections) of a marine trematode parasite, Himasthla elongata (Trematoda: Echinostomatidae), on infection intensity and success in its second intermediate host, the blue mussel Mytilus edulis, an abundant and widely distributed bivalve in European coastal waters. In our laboratory experiment, we tested 4 levels of parasite doses and showed that mussels faced higher parasite infection intensity at higher doses of cercarial exposure and that they acquired more infections when repeatedly exposed to smaller doses compared to a single high dose. However, the infection success of cercariae did not differ among 4 dose levels but was only significantly different between trickle and single exposures. This indicates that cercariae were not subjected to a dose-dependent regulation of their infectivity, suggesting that infection intensity in mussels is largely driven by factors mediating the abundance of infective stages. With the combined investigation of the effect of cercarial dose and exposure frequency at realistic dose levels, our study contributes to our currently very limited understanding of the determinants of infection intensity in marine hosts and highlights the usefulness of experimental studies in advancing our knowledge in this field.

Parasites as prey: the effect of cercarial density and alternative prey on consumption of cercariae by four non-host species.

In parasites with complex life cycles the transmission of free-living infective stages can be influenced by ambient community diversity, in particular via predation. Here, we experimentally investigated whether parasite density and the presence of alternative prey can alter predation rates on free-living cercarial stages of a marine trematode by several non-host predators. All four predator species consumed increasing numbers of cercariae with an increase in cercarial density, indicating that the removal of cercariae by predators is effective over a range of natural densities as well as in the presence of alternative prey for a number of predators typical of marine ecosystems. However, the relative removal rates and the effects of cercarial density and alternative prey differed among predator species. In barnacles and shrimps, significant interactive effects of cercarial density and alternative prey on cercarial predation occurred while in oysters and crabs cercarial removal rates were unaffected by both factors. As changes in cercarial densities directly translate into changes in infection levels in down-stream hosts in this parasite-host system, the observed predator-specific responses suggest that cercarial predation effects on disease risks will depend on the specific species composition of ambient communities and not on non-host biodiversity per se.

Age and serotonin effects on locomotion in marine trematode cercariae

The effect of serotonin solutions on the locomotor activity of cercariae with different swimming patterns, Cryptocotyle lingua and Himasthla elongate, was investigated during their lifespan using motion track analysis and a frame-by-frame count of behavioral elements. Serotonin caused a noticeable alteration of several locomotion parameters in both species studied, while the activity per se decreased significantly with age. In intermittently swimming C. lingua cercariae, serotonin induced an increase in the frequency of active swimming phases (spurts) without significant changes in the duration and velocity of the spurts themselves. In continuously swimming H. elongata cercariae, serotonin increased the number of stops during the first hour after the emission from the host mollusk under a constant average swimming velocity. Immunocytochemical visualization of serotonin revealed a decline in its level with age (time after emission) and different dynamics of this process in neurons located in the tail and body of C. lingua cercariae. The data are interpreted in the light of the modulatory role of serotonin known for other animals.

Trematodes on acid: editorial comment on the feature article by Guilloteau et al.

metacercariae, awaiting ingestion by the definitive host. For the first time, Guilloteau and co-authors studied how OA would alter the performance of infective stages within snails. They found that lower pH increased the production of cercariae in both species, with a possible trade-off of smaller sizes over a 2-month experimental period. The longevity of encysted metacercariae assessed over a 30-day interval was reduced in the one (Philophthalmus) but not in the other species (Parorchis). These strong speciesspecific effects highlighted that currently, no generalizations are possible with respect to OA effects on marine trematodes. As a fascinating zoological curiosity, their model species Philophthalmus and Parorchis display caste diversification. Some individuals develop into smaller morphotypes that defend the larger clonal proliferators against competing trematode species (so-called soldiers). As a result of experimental ocean acidification, the cast ratios were biased towards fewer defensive stages, again an effect that was species specific. Philophthalmus showed the highest proportion of small ‘soldier’ morphotypes at intermediate acidification levels, while in Parorchis, fewer defensive stages were found at the lowest pH. The observed effects most likely have consequences for parasite transmission rates and hence for the prevalence of these species within the diverse host species. Taken together these results highlight both the need to address how the inevitable progression of dissolved CO2 into the ocean as a result of anthropogenic burning of fossil fuels alters species interactions beyond competition (James et al. 2014), grazing (Borell et al. 2013) and predation (Kroeker et al. 2014), namely the neglected field of host– parasite interactions. Another valuable next step would be to assess the long-term evolutionary consequences of acidification on the equilibrium of hosts and parasites (Sunday The field of ocean acidification (OA) research is moving rapidly, moving from studying first-order direct effects of lowered pH and carbonate ion concentration impinging directly upon organismal physiology to ask how species interactions are modified. How host–parasite interactions may be modulated by ocean acidification is currently largely unknown, in particular for macroparasites (=metazoan animal parasites). This is unfortunate, given that these interactions are among the strongest biological interactions in all ecosystems, while their food web role, in particular in coastal systems, is often neglected (but see Kuris et al. 2008). Digenean trematodes are one of the most abundant and diversified groups of macroparasites worldwide, displaying a breathtaking diversity of twoor three-phasic life cycles (Galaktionov and Dobrovolskij 2003). Often, the first intermediate host is a mollusc (snail or bivalve), followed by a fish as second intermediate host, and a mammal or bird species as third and definitive host where sexual reproduction takes place. In this issue of Marine Biology, the study by Guilloteau et al. (2016) focused on the mollusc host phase where the infective parasite stages rapidly multiply via clonal proliferation, a critical step for generating millions of free-swimming cercariae. Note that in the case of the two model species in the study by Guilloteau et al. (2016) cercariae leave out the 2nd intermediate host and encyst directly on benthic surfaces to become

Impacts of ocean acidification on multiplication and caste organisation of parasitic trematodes in their gastropod host

Ocean acidification is predicted to impact the structure and function of all marine ecosystems in this century. As focus turns towards possible impacts on interactions among marine organisms, its effects on the biology and transmission potential of marine parasites must be evaluated. In the present study, we investigate two marine trematode species (Philophthalmus sp. and Parorchis sp., both in the family Philophthalmidae) infecting two marine gastropods. These trematodes are unusual in that their asexually multiplying stages within snails display a division of labour, with two distinct castes, a large-bodied morph producing infective stages and a smaller morph playing a defensive role against other competing parasites. Using a potentiometric ocean acidification simulation system, we test the impacts of acidified seawater (7.8 and 7.6 pH) on the production of free-living infective stages (cercariae), the size and survival of encysted resting stages (metacercariae), and the within-host division of labour measured as the ratio between numbers of the two morphs. In general, low pH conditions caused an increase in cercarial production and a reduction in metacercarial survival. The ratio of the two castes within snail hosts tended to shift towards more of the smaller defensive morphs under low pH. However, the observed effects of reduced pH were species specific and not always unimodal. These results suggest that ocean acidification can affect the biology of marine parasites and may also impact transmission success and parasite abundance of some trematodes, with possible consequences for marine communities and ecosystems.

Social flatworms: the minor caste is adapted for attacking competing parasites

Recently, it has been hypothesized that some marine parasitic flatworms (trematoda) within their gastropod hosts form socially organized larval colonies divided into distinct castes adapted for defence and reproduction. However, the evidence in support for advanced division of labour in trematodes is challengeable and generally consistent with the traditional view of ordinary age-structured colonies. Using intramolluscan larval colonies of the trematode Himasthla elongata (Echinostomatidae) as study system, we demonstrate experimentally that contrary to reproducing colony members, non-reproducing larvae are anatomically and behaviourally adapted of defending the colony against competing parasites infecting the same host. We also show that co-infections are significantly less frequent in situ than expected by chance, which emphasizes the efficiency of the antagonistic activity of Himasthla colonies in eliminating competitors. Together, this brings novel and pivotal support to the view of adaptive cast formation in marine trematodes.

Differential tolerances to ocean acidification by parasites that share the same host.

Ocean acidification is predicted to cause major changes in marine ecosystem structure and function over the next century, as species-specific tolerances to acidified seawater may alter previously stable relationships between coexisting organisms. Such differential tolerances could affect marine host-parasite associations, as either host or parasite may prove more susceptible to the stressors associated with ocean acidification. Despite their important role in many ecological processes, parasites have not been studied in the context of ocean acidification. We tested the effects of low pH seawater on the cercariae and, where possible, the metacercariae of four species of marine trematode parasite. Acidified seawater (pH 7.6 and 7.4, 12.5 °C) caused a 40-60% reduction in cercarial longevity and a 0-78% reduction in metacercarial survival. However, the reduction in longevity and survival varied distinctly between parasite taxa, indicating that the effects of reduced pH may be species-specific. These results suggest that ocean acidification has the potential to reduce the transmission success of many trematode species, decrease parasite abundance and alter the fundamental regulatory role of multi-host parasites in marine ecosystems.

The most vagile host as the main determinant of population connectivity in marine macroparasites

Although molecular ecology of macroparasites is still in its infancy, general patterns are beginning to emerge, e.g. that the most vagile host in a complex life cycle is the main deter- minant of the population genetic structure of their parasites. This insight stems from the observa- tion that populations of parasites with only freshwater hosts are more structured than those with terrestrial or airborne hosts. Until now, the same has not been tested for marine systems, where, in theory, a fully marine life cycle might sustain high dispersal rates because of the absence of obvi- ous physical barriers in the sea. Here, we tested whether a marine trematode parasite that utilises migratory birds exhibited weaker population genetic structure than those whose life cycle utilises marine fish as the vagile host. Part of the mitochondrial cytochrome c oxidase 1 (COI) gene was sequenced from individual sporocysts from populations along the Atlantic coast of Europe and North Africa. Strong population structure (Φst = 0.25, p < 0.0001) was found in the fully marine trematode Bucephalus minimus (hosted by fish), while no significant structure (Φst = 0.015, p = 0.19257) was detected in Gymnophallus choledochus (hosted by birds). However, demographic models indicate recent colonisation rather than high dispersal as an alternative explanation of the low levels of structure observed in G. choledochus. Our study is the first to identify significant genetic population structure in a marine autogenic parasite, suggesting that connectivity between populations of marine parasites can be limited despite the general potential for high dispersal of their hosts in the marine environment.

Inventory of organisms interfering with transmission of a marine trematode

It has increasingly been recognized that organisms can interfere with parasitic free-living stages, preventing them from infecting their specified host and thus reducing infection levels. This common phenomenon in freshwater and terrestrial systems has been termed the ‘dilution effect’ and, so far, is poorly studied in marine systems. Ten common intertidal organisms found in the Dutch Wadden Sea (North Sea) were tested to establish their effects on the free-living cercarial stages of the trematode parasite Himasthla elongata. Most species tested resulted in a significant reduction in cercariae over a 3 hr time period. The amphipod Gammarus marinus removed 100% of the cercariae, while other effective diluters were Crangon crangon (93%), Sargassum muticum (87%), Semibalanus balanoides (71%), Crassostrea gigas (67%), Hemigrapsus takanoi (&gt;54%), Crassostrea gigas shells (44%) and Idotea balthica (24%). In contrast, mixed shells (Cerastoderma edule, Mytilus edulis, Ensis americanus and Littorina littorea) and Fucus versiculosus had no significant effect. These results suggest that dilution effects are widespread in the trematode of H. elongata, with potentially strong effects on its population dynamics.

Does a facultative precocious life cycle predispose the marine trematode Proctoeces cf. lintoni to inbreeding and genetic differentiation among host species?

Intraspecific variability in parasite life cycle complexity (number of hosts and species of hosts in the life cycle) may have an impact how parasite genetic variation is partitioned among individual parasites, host individuals or host species within a given area. Among digenean trematodes, a three-host life cycle is common. However, a few species are precocious and may reach sexual maturity in what is typically regarded as the second intermediate host. The objective of this study was to determine whether a precocious life cycle predisposes digeneans to possible inbreeding or genetic subdivision among host species. As a study system, we used the digenean Proctoeces cf. lintoni whose metacercariae precociously mature (facultative) without a cyst wall in the gonads of multiple sympatric species of keyhole limpets (Fissurella spp.), typically regarded as the second intermediate hosts. Genotyped parasites were collected from four species of limpets and the clingfish Sicyases sanguineus, the third and final host where sexual maturity occurs. We found very high microsatellite diversity, Hardy–Weinberg equilibrium over all genotyped individuals, and little to no genetic structuring among parasites collected from the different host species. The fact that metacercariae do not encyst in the keyhole limpets, coupled with the high mixing potential of an aquatic environment, likely promote panmixia in local populations of P. cf. lintoni.

In vitro culture of marine trematodes from their snail first intermediate host

The ability to culture parasites outside their host (i.e. in vitro) is essential for several aspects of parasitological research. Here, a culture medium for marine trematode parthenitae was optimized using Philophthalmus sp. rediae from the intermediate snail host, Zeacumantus subcarinatus. The medium was optimized by sequentially testing the suitability of different levels of osmolality, different commercially available media, and different concentrations of supplemented chicken serum, while controlling for genetic variation among cultures. Philophthalmus sp. rediae survived up to 56days in cultures of the best tested medium, remaining active and continuously shedding cercariae. The broader suitability of the culture medium was tested using five other trematode species from different families (using either the same or other marine snails as first intermediate hosts): Galactosomum sp., Acanthoparyphium sp., Maritrema novaezealandensis, Curtuteria australis, and an undescribed species of the family Opecoelidae. Survivorship of rediae and sporocysts from these species ranged from eight days to 42days. The culture procedures developed here can therefore be used in the future as a system under which to culture marine trematode parthenitae for experimental studies.